Field of the Invention
The present invention relates to a dispersion comprising carbon nanotubes and graphene platelets and having a particular mass ratio of these components. It further relates to a process for producing such a dispersion, the use thereof as printing ink for producing electrically conductive films and an electrically conductive film based thereon.
Description of Related Art
Printed electronics are being increasingly used in a variety of applications such as portable electronics, signs, illumination, product identification, flexible electronics (which can, for example, be rolled or deformed), photovoltaics, medical apparatuses, antennas (in particular RFID antennas), displays, sensors, thin film batteries, electrodes and many others. Printed electronics have a variety of advantages over conventional electronics. Printing of conductive structures can be quicker than substractive processes (such as etching), produce less waste and involve a smaller amount of hazardous chemicals than in conventional processes. The resulting electronics can be used more simply in flexible applications such as flexible displays which can be rolled, twisted, bent or otherwise deformed.
Printed electronics are usually made by printing the conductor tracks or other constituents of the electric circuits onto a substrate using an electrically conductive, metallic ink. The inks usually contain silver particles and occasionally copper particles, other metal particles and/or conductive polymers. However, conductive polymers alone are generally not sufficiently electrically conductive. In addition, the resulting printed metallic circuits are only inadequately electrically conductive in flexible applications in which they are regularly mechanically deformed by bending and/or stretching.
The printed substrates frequently have to be subsequently sintered at elevated temperatures in order to join the conductive metal particles to one another so as to achieve the desired electrical conductivity. The temperatures required for the sintering processes often restrict the choice of substrates for producing the electronics. While inexpensive materials such as paper, polyolefins (for example polypropylene) and the like would be desirable as substrates for printed electronics in numerous applications, the sintering temperatures often required are too high for such materials to be able to be used. In addition, silver is expensive, while base metals can form oxides on exposure to air and these make the material insufficiently conductive for the respective use.
In addition, the use of metal-based inks can contribute to an increased weight of the resulting component and due to the abovementioned sintering process can add one or more additional steps, time and complexity to the production process. It would therefore be desirable to achieve printed electronic components using inks which do not contain expensive noble metals.
In the light of this background, US 2007/0284557 A1 and US 2009/0017211 A1 disclose a transparent and conductive film which comprises a network of graphene platelets. The film can also contain an intermeshing network of other nanostructures, a polymer and/or a functionalizing agent. This patent application further describes a process for producing such a film, for example by provision of the graphene platelets in a solution and removal of the solvent. These patent applications also disclose, in an example, graphene-CNT composites as constituents of the film, but without giving more detailed information regarding the relative proportions of graphene and CNT.
US 2007/0158610 A1 relates to a process for producing a stable suspension of carbon nanoparticles in a hydrophilic heat transfer liquid in order to improve the thermal conductivities and other properties such as the freezing point of an antifreeze. The process comprises the step of dispersing carbon nanoparticles directly in a mixture of a heat transfer liquid and other additives in the presence of surfactants under the temporary action of ultrasound.
US 2010/0000441 A1 is concerned with a conductive ink which is based on graphene platelets and comprises (a) graphene nanoplatelets (preferably unoxidized or original graphene) and (b) a liquid medium in which the graphene nanoplatelets are dispersed, where the graphene nanoplatelets take up at least 0.001% by volume, based on the total volume of the ink. The ink can also contain a binder or matrix material and/or a surfactant. Furthermore, the ink can contain other fillers such as carbon nanotubes, carbon nanofibers, metal nanoparticles, carbon black, conductive organic compounds, etc. The graphene platelets preferably have an average thickness of not more than 10 nm and preferably of not more than 1 nm. Inks mentioned can be printed in order to form electrically or thermally conductive components. An example mentions an ink containing 5% of carbon nanotubes and 1% of graphene nanoplatelets.